1. Field of the Invention
The present invention relates to an optical code division multiple access (OCDMA) system, and more particularly to a two-dimensional OCDMA system sequentially encoding data bits in time domain and in the wavelength domain using two independent pseudo random noise (PN) sequences, and decoding the encoded results in the wavelength and time domains based on wavelength difference detection (or equal detection of wavelength) such that multiple access interference (MAI) among concurrent users can be removed, and to a wavelength/time sequential encoder and decoder adopted in the OCDMA system. Also the present invention relates to a method of encoding/decoding data bits/decoded results using the same.
2. Description of the Related Art
Generally, research into performance enhancement of an OCDMA system have been proceeded to increase the number of creation code and to reduce noise including MAI among concurrent users. To achieve the above objectives, a method for increasing created code dimension was proposed.
As a prior patent for an encoding/decoding technique of the prior art OCDMA system, U.S. Pat. No. 6,614,950, entitled FIBER BRAGG GRATING-BASED OCDMA ENCODER/DECODER, whose assignee is the National Science Council, discloses a wavelength domain OCDMA system using PN code in one-dimension. In the U.S. patent, the encoder includes a broadband light source, a fiber Bragg grating (FBG) array, and an optical circulator. The FBG array reflects a specific wavelength according to previously allocated PN code and transmits it to a network through the optical circulator to perform wavelength domain encoding. The encoder performs an encoding operation if the data bit is ‘1’ and does not transmit a signal thereto if the data bit is ‘0.’ Also, the decoder includes a pair of FBG arrays, composed according to allocated codes and codes complementary to the allocated codes, and two optical detectors performing wavelength difference detection. Lights passing through the pair of FBG arrays are incident upon the respective optical diodes. If the lights correspond to the codes of the FBG arrays, they are incident upon one photodiode to output a signal ‘1.’ However, if the lights correspond to codes of another users, they are divided to be incident upon two photodiodes. On the other hand, since the length of PN codes used in the prior U.S. patent is odd, MAI occurs. Also, since the encoding and decoding are performed in the one-dimensional wavelength domain, the code number is limited.
In order to overcome the problems in the prior U.S. patent, two dimensional encoding/decoding techniques have been proposed. Namely, a system adopting the prior art encoding/decoding techniques composes a new two-dimensional code created as prime code is combined with optical orthogonal code, and transmits signals based on the combination thereto as FBG arrays corresponding to two pair of Single Pulse Per Row type codes corresponding to ‘1’ and ‘0’ are switched by control signals, respectively. However, the proposed system has disadvantages in that the created code number is limited and, furthermore the subscriber number cannot be secured since two codes are distributed to a single subscriber.
Uri N. Griner and Shlomi Arnon disclosed two-dimensional encoding/decoding techniques [IEEE, Photonics Technology Lett., Vol. 16, No. 1, pp. 332˜334, January 2003, titled “A novel bipolar wavelength-time coding scheme for optical CDMA systems”], using bipolar time spreading of a broadband light source and complementary spectral-amplitude-coding, in which two LEDs perform a coding operation in the time domain according to signals ‘1’ and ‘−1.’ After that, another coding operation is performed in the wavelength domain through a diffraction grating. In a receiving unit, wavelength difference detection is performed by two photodiodes to retrieve the signals. The above system employs a Hadamard code for performing a coding operation in the wavelength domain, and a barker code of length, 3, 7 and 11 for performing a coding operation in the time domain. However, the prior art system has disadvantage in that MAI cannot be completely removed therefrom due to inherent characteristics of the barker code adopted in the time domain.